Conventionally, a hydraulic drive system for a working machine has been designed to permit detecting clogging of a filter in an exhaust gas purification device. When the working machine is in a non-operated state at the time of detection of clogging, the hydraulic drive system automatically performs both raising the delivery pressure of a hydraulic pump and increasing the delivery flow rate of the hydraulic pump in parallel, whereby an engine output is increased. This increase in engine output leads to a rise in the temperature of exhaust gas. When the temperature of the exhaust gas rises to a temperature needed for the burning of particulate matter, the particulate matter with which the filter is clogged burns off (see Patent Document 1).